#include <asf.h>

void usart_clock_init (void);
void usart_pin_init (void);

static inline void pin_set_peripheral_function (uint32_t pinmux);

static inline void pin_set_peripheral_function (uint32_t pinmux)
{
  uint8_t port = (uint8_t)((pinmux >> 16)/32);
  PORT->Group[port].PINCFG[((pinmux >> 16) - (port*32))].bit.PMUXEN = 1;
  PORT->Group[port].PMUX[((pinmux >> 16) - (port*32))/2].reg &= ~(0xF << (4 * ((pinmux >> 16) & 0x01u)));
  PORT->Group[port].PMUX[((pinmux >> 16) - (port*32))/2].reg |= (uint8_t)((pinmux & 0x0000FFFF) << (4 * ((pinmux >> 16) & 0x01u)));
}

void usart_clock_init (void)
{
  struct system_gclk_chan_config gclk_chan_conf;
  uint32_t gclk_index = SERCOM3_GCLK_ID_CORE;
  system_apb_clock_set_mask(SYSTEM_CLOCK_APB_APBC, PM_APBCMASK_SERCOM3);
  system_gclk_chan_get_config_defaults(&gclk_chan_conf);
  system_gclk_chan_set_config(gclk_index, &gclk_chan_conf);
  system_gclk_chan_enable(gclk_index);
}

void usart_pin_init (void)
{
  pin_set_peripheral_function(PINMUX_PA20D_SERCOM3_PAD2);
  pin_set_peripheral_function(PINMUX_PA21D_SERCOM3_PAD3);
}

int main (void)
{
  system_init();
  usart_clock_init();
  usart_pin_init();

  struct usart_config usart_conf;
  struct usart_module usart_mod;
  usart_get_config_defaults (&usart_conf);
  stdio_serial_init(&usart_mod, SERCOM3, &usart_conf);
  usart_enable(&usart_mod);

  usart_write_wait(&usart_mod, (uint16_t)'a');
  usart_write_wait(&usart_mod, (uint16_t)'b');
  usart_write_wait(&usart_mod, (uint16_t)'c');
  usart_write_wait(&usart_mod, (uint16_t)'d');
  usart_write_wait(&usart_mod, (uint16_t)'e');
  usart_write_wait(&usart_mod, (uint16_t)'f');
  usart_write_wait(&usart_mod, (uint16_t)'g');
  usart_write_wait(&usart_mod, (uint16_t)'h');
  usart_write_wait(&usart_mod, (uint16_t)'i');
  usart_write_wait(&usart_mod, (uint16_t)'j');
  usart_write_wait(&usart_mod, (uint16_t)'k');
  usart_write_wait(&usart_mod, (uint16_t)'l');
  usart_write_wait(&usart_mod, (uint16_t)'m');
  usart_write_wait(&usart_mod, (uint16_t)'n');
  usart_write_wait(&usart_mod, (uint16_t)'o');

}